Process control systems have been implemented for many years and across various industries. In petroleum processing, power generation, and chemical manufacturing, process control systems have been utilized to varying degrees, from small control systems having a few input/output (I/O) nodes to vastly larger systems having hundreds, and even thousands, of the field instruments and devices, such as, for example, valves, louvers, regulators, displacement and float level sensors, relief valves, and alarms.
In many process control systems, it is common to have one or more field devices implemented in a stand-alone snap-acting application. In this configuration, the field device is essentially separate from the control system in that the field device can only be controlled or monitored by control system personnel at the site of the field device. The secluded or remote field device is considered to be operating “blind,” as the device does not provide feedback to the process control system pertaining to operating information. Such feedback, may typically consist of, for example, the actual position of a valve or other information regarding the valve.
In the past, these “blind,” field devices which often are situated in remote locations, were typically controlled via a pneumatic or hydraulic controller. Pneumatic controllers tend to be well suited for the automation of simple repetitive tasks. However, such controllers may be subject to mechanical fatigue and deterioration, which can adversely affect both the accuracy and the repeatability of the process control system, either of which may ultimately lead to failure of the control system. It would therefore be beneficial to have a field device that can be remotely controlled and/or monitored, so the process control system can more readily assess the operating condition of the field device.
FIG. 1 depicts a common configuration of a local control loop in a stand-alone application. An event trigger associated with the process is monitored by a sensor such as, for example, a pressure switch, a micro switch, a limit switch, or other suitable devices. The sensor is connected to the controller, and in response to an occurrence of the event trigger, the sensor alerts the controller. In turn, the controller energizes an actuator or transducer associated with the field device to adjust the process. For example, the controller may cause the transducer or actuator to move a control valve, flip a switch, or raise or lower a temperature or pressure associated with the process.
Initially, pneumatic control systems did not incorporate programmable logic controllers (PLC) or any electrical/electronic controllers. Control systems eventually utilized digital logic with the incorporation of solid-state devices, programmable logic arrays (PLAs), PLCs, and microprocessors and microcontrollers. With the advent of such electronic control systems, it became possible to remotely monitor and control field devices via wired-communication between the controller and the field device.
If there was a desire to remotely monitor the field device of a stand-alone control loop such as that shown in FIG. 1, the sensor and field device could be wired to the electronic control system. Such a configuration is show in FIG. 2 wherein the local control loop of FIG. 1 utilizes a control system. In this arrangement, a process value or variable is sent from the sensor to the control system via a communication protocol such as Fieldbus™. The control system then determines a set-point (for example, an operational parameter) and sends a control signal over a communication bus to the field device to control the process. Thus, responsibility for monitoring and controlling the stand-alone field device was relocated from the site of the field device to a distant or remote control system.
One concern with wire-coupling a control loop to a control system is the cost and effort associated with the actual wiring of the system. Often times it simply is not feasible to hard-wire a control loop to a control system because of the distance, inhospitable terrain, dangerous environment, or easement restrictions between the two locations. Another concern is the increase in time involved to control the remotely connected field device as compared to the initial stand-alone configuration and its snap-acting operation. That is, the time required to send a control response to the wired field device includes time for sending the sensed process value to the remote control system, processing the sensed value at the controller, and sending the control command back to the remotely located control element, as well as the time it takes for the control element to react. This increase in time may adversely affect the ability of the system to control processes where the monitored variable changes quickly.
To lessen the cost and complexity associated with wiring remote field devices to the control system, some portions of the control process were modified for wireless communication. A typical implementation of a wireless sensor is shown in FIG. 3, wherein the wiring between the sensor and the control system shown in FIGS. 1 and 2 has been replaced with wireless capability. This modification however does not address the concerns that arise relating to the associated increase in time to control the field device compared with respect to the snap-acting configuration. On the contrary, because the wireless sensor in many cases is battery powered, the rate of transmission from the wireless sensor to the control system is usually reduced to conserve the battery. The minimized transmissions of the sensor value to the control system unfortunately may further extend the time needed to control the process as compared to systems utilizing wired sensors.
In view of the above concerns that need to be addressed for remotely controlling and/or monitoring a field device, it is not difficult to understand why many field devices remain configured in stand-alone applications. Controlling such field devices therefore still requires control personnel to visit the site of the field device, which may be located in a harsh or hazardous environment, to modify or adjust the state, position, or other operating parameters of the field device.